Morphine has long been known as a powerful analgesic having a morphinan skeleton, and is widely used even at present. However, this drug has serious side effects that present clinical problems, including drug dependence, action that suppresses respiration and action that suppresses smooth muscle movement (constipation). Thus, there is a need for a powerful analgesic that acts on the central nervous system and that also can be used safely.
In addition, it has also been reported that drugs that act on opioid receptors effect urination (J. D. Leander, J. Pharmacol. Exp. Ther., 227, 35 (1983)), and thus, the effective use of that action is also desired.